The present invention relates to a spindle motor and rotating shaft supporting device for a spindle motor equipped in information equipment. Information equipment referred to herein includes information processing devices such as a laser beam printer (LBP), a digital facsimile, a digital duplicating machine (PPC), etc., in addition to information recording equipment using an information recording carrier such as a magnetic disk unit (HDD, FDD), an optical disk unit (CD, DVD), a photomagnetic disk unit (MD, MO, ODD), a digital audio tape recorder (DAT).
The spindle motor of abovementioned information equipment is required, in addition to having high precision of rotation, to be capable of rotating at a high speed, to be low in cost and to lessen noise during operation. One of factors which determines whether these requirements are satisfied or not is a supporting device that supports the spindle of the motor. In the prior art a ball bearing or an oil-impregnated wintered bearing has been used for the supporting device.
Use of the ball bearing, however, results in drawbacks as described below.
Such spindle motors are often operated at high rotational speeds from about 5000 to 12000 rpm, in particular, tens thousands of revolutions per minute in a laser beam printer. A ball bearing generates characteristic racing noise (sound generated by balls rolling on a bearing ring) and other noise due to self-excited vibration of a cage. The noise level increases as the operating speed increases, and attempts to reduce the noise has been limited. Moreover, high rotation accuracy is required in the abovementioned spindle motor to cope with RRO (Repeatable Run Out), NRRO (Non Repeatable Run Out), jitter, etc. There are many components such as an outer ring, inner zing, balls, a cage, seal, grease, etc., in a ball bearing, and there are many factors such as mechanical accuracy, assembling accuracy onto the spindle motor, preloading method, etc., which influence the rotation accuracy, and are complicatedly combined. Therefore, it is difficult to control accuracy. Even though accuracy is controlled, costs are increased.
An oil-impregnated sintered bearing, on the other hand, has such advantages over the ball bearing as generating less operating noise and comprising less component parts resulting in lower cost, but has drawbacks as descried below.
To operate optical disk unit, a user selects a particular disk from among a plurality of disks and sets the selected disk on a turn table. However, since there are variations in the manufacturing accuracy among individual disks, unbalanced load caused when the motor to rotate, results in a RRO of the rotating shaft (disk rotating out of horizontal plane). In the conventional oil-impregnated sintered bearing, loaded region moves in the circumferential direction as the shaft rotates when used under rotation at such a high speed as described above, while the oil film cannot follow the movement thus resulting in the failure of oil film formation. This phenomenon causes increase of the RRO of the rotating shaft (disk rotating out of horizontal plane) and furthermore develops in various drawbacks such as miss-read of the disk. Also in the oil-impregnated sintered bearing, air is entrained into the bearing during rotation and the amount of entrained air increases as the rotation speed increases, thus hindering the formation of the oil film. When oil film is not formed sufficiently, metal contact takes place thereby accelerating wear. The wear causes the RRO to grow which makes it difficult for the oil film to form, thus falling into a vicious cycle and imposing a problem in terms of durability. Furthermore, unstable vibration such as a whirl occurs if the oil-impregnated sintered bearing is used for BDD, LBP, etc., since it is a type of cylindrical plain journal bearing. In such circumstances, it cannot be achieved accuracy such as NRRO requited in above device.
The present invention aims at solving the problems of the ball bearing a-ad the oil-impregnated sintered bearing described above, and it is an object of the invention to provide a spindle motor and a rotating shaft support device thereof for information equipment.